This invention relates to molding of solid pellets from a semi-solid material. More specifically, the invention concerns a machine in which a mud composition is injected into mold openings in a rotating disk, and the composition is dried to produce solid pellets.
Acrolein is a chemical compound useful for producing various intermediate compounds, such as acrylic acid, acrylonitrile, glycerine and methionine. In a well known process for producing acrolein, the product is obtained from the vapor phase oxidation of propylene in the presence of a suitable oxidation catalyst. A recent improvement in a catalyst composition used in the manufacture of acrolein is described in U.S. patent application Ser. No. 640,616, filed Dec. 15, 1975, now U.S. Pat. No. 4,049,577. Briefly, the oxidation catalyst described in this reference is prepared by mixing certain metal salts, such as oxides of cobalt, iron, bismuth, molybdenum and potassium, in a water solution.
The resulting slurry is a thixotropic mixture which contains about 65 percent water, by weight. The slurry is then dried by mechanical means to reduce the moisture content to about 10-30 percent water, by weight. After drying, this composition is in the form of a light clay which can be molded to produce solid products, such as spherical or cylindrical pellets. The pellets are then calcined to produce the oxide form of the catalyst material.
Although a spherical shape is preferred, the oxidation catalyst is usually molded in the form of a cylindrical pellet, since it is easier to mold the material in a cylindrical shape. In the molding operation it is essential to produce catalyst pellets which have a uniform shape and size. For example, if the pellets are of irregular shapes and sizes the pellets will not stack properly in the catalyst bed in the reactor. The result can be a catalyst bed which does not have sufficient void passages between the pellets to achieve a good vapor phase oxidation of the propylene starting material.
Extrusion molding and injection molding are examples of techniques which have been used in the past to produce the catalyst pellets. Neither of these methods has been satisfactory for producing pellets of a uniform size and shape. In a typical extraction molding procedure the light clay, which is a soft composition, is forced through a circular die hole. The material extruded through the die hole forms a cylindrical strand, which is cut into short pieces. It is difficult to control the feed rate of the strand, and the result is that the length of each piece may vary considerably. Another problem is that when the soft strand is cut into short pieces, the cylindrical pellets frequently have jagged edges when dried. When the pellets are placed in the reactor the jagged edges will flake off and create a dust problem in the reactor.
In a typical injection molding procedure a screw conveyor forces the soft clay composition into a two-piece die. Each die face has a hemispherical shape, so that the complete mold forms a spherical pellet. The pellet is dried by heaters positioned in the mold. When the mold is opened the spherical pellets will sometimes rupture at the seam. This is due to a sudden release of steam from the inside of the pellet. An additional problem is that the pellets will frequently stick to the die face, so that they are damaged when removed from the mold.